As there is a changing demand for mobile communication traffic, a quantity of organizations including the ITU, etc., have defined a shorter user-plane delay as required for future mobile communication systems. A common practice to shorten a user-plane delay is to reduce a length of a Transmission Time Interval (TTI). After the TTI of a traffic channel is reduced, there has been absent so far a solution to how to transmit Acknowledgement (ACK)/Non-Acknowledgment (NACK) feedback information of downlink data.
The existing LTE frame structures (in the LTE Rel-8/9/10/11/12/13) will be introduced as follows.
In a Frame Structure Type 1 (FS1) of an existing LTE FDD system, there are different carrier frequencies for uplink and downlink transmission, and there is the same frame structure for both uplink and downlink transmission. Where a radio frame with a length of 10 ms over each carrier includes ten 1 ms sub-frames and each sub-frame includes two slots with a length of 0.5 ms. Where a length of time of a TTI for uplink and downlink data is 1 ms.
In a Frame Structure Type 2 (FS2) of an existing LTE TDD system, there are different sub-frames or slots at the same frequency for uplink and downlink transmission. In the FS2, each radio frame with a length of 10 ms includes two half-frames with a length of 5 ms, and each half-frame includes five sub-frames with a length of 1 ms. The sub-frames in the FS2 are categorized into downlink sub-frames, uplink sub-frames, and special sub-frames, where each special sub-frame includes three components of a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS). Where a downlink pilot, downlink traffic data, and downlink control signaling can be transmitted in the DwPTS; no signal is transmitted in the GP; and only a random access signal and a Sounding Reference Symbol (SRS) can be transmitted, but neither uplink traffic data nor uplink control information can be transmitted, in the UpPTS. Each half-frame includes at least one downlink sub-frame, at least one uplink sub-frame, and at most one special sub-frame. Table 1 depicts seven uplink-downlink sub-frame configurations supported in the FS2.
TABLE 1Uplink-downlink configurationsUplink-Downlink-downlinkto-UplinkConfig-Switch-pointSub-frame numberurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
ACK/NACK feedback timing in the existing LTE FDD/TDD system will be introduced as follows.
In the LTE Rel-8 system, for the FDD system, each sub-frame has an uplink time domain resource, and ACK/NACK feedback information of a downlink sub-frame with the number n is transmitted in an uplink sub-frame with the number n+4. For the TDD system, frequency resources are shared for uplink and downlink transmission, so sub-frames shall be categorized into uplink and downlink sub-frames as depicted in Table 1, so that ACK/NACK feedback information of different downlink sub-frames may have to be transmitted in the same uplink sub-frame, and as depicted in Table 2, downlink sub-frames for which ACK/NACK is to be fed back in an uplink sub-frame with the number n (i.e., a downlink sub-frame with the number n−k, including a special sub-frame) can be determined according to respective elements in a set K: {k0, k1, . . . kM-1} corresponding to the uplink sub-frame, so ACK/NACK feedback information of M downlink sub-frames shall be fed back in the same uplink sub-frame, where M is the quantity of elements in the set K, and may take different values for different uplink sub-frames and different TDD uplink-downlink configurations.
TABLE 2Downlink association set index K: {k0, k1, . . . kM−1} for the TDD systemUL-DLSub-frame nConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7,——4, 63——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5,——————4, 75——13, 12, 9, 8, 7, 5, 4, 11, 6———————6——775——77—
It shall be noted that the numbers in Table 2 are set in the unit of a radio frame, and if the sub-frame number n−k is less than zero, for example, then it will indicate that the sub-frame belongs to a preceding radio frame.
When carriers are aggregated, a terminal can operate concurrently over a plurality of carriers, but can only transmit ACK/NACK feedback information in a Physical Uplink Control Channel (PUCCH) over a part of the carriers, e.g., over a Primary Component Carrier (PCC). Accordingly the terminal further needs to feed back ACK/NACK feedback information corresponding to the plurality of carriers, in an uplink sub-frame. When all the carriers aggregated for the terminal are FDD carriers, ACK/NACK feedback timing over each carrier is determined as per n+4; and when all the carriers aggregated for the terminal are TDD carriers, if the same TDD uplink-downlink configuration is applied to all the TDD carriers, then ACK/NACK feedback timing over each carrier will be determined according to the feedback timing corresponding to the TDD uplink-downlink configuration in Table 2. If there are different TDD uplink-downlink configurations of the aggregated TDD carriers, then a reference TDD uplink-downlink configuration will be defined for a Secondary Component Carrier (SCC) under a specific rule, and ACK/NACK feedback will be made according to the feedback timing corresponding to the reference TDD uplink-downlink configuration in Table 2; and when the carriers aggregated for the terminal include both the FDD carrier(s) and the TDD carrier(s), if one of the FDD carrier(s) is a PCC, then ACK/NACK feedback timing over each carrier will be determined as per n+4, and if one of the TDD carrier(s) is a PCC, then a reference TDD uplink-downlink configuration will be defined for an SCC under a specific rule, and ACK/NACK feedback will be made according to the feedback timing corresponding to the reference TDD uplink-downlink configuration in Table 2. In a subsequent LTE release, aggregated carriers can be further grouped into a plurality of carrier groups, and ACK/NACK feedback can be made in a PUCCH over one of the carriers in each carrier group; and at this time, feedback timing of an SCC in each carrier group can be determined according to one of the above instances that correspond to the aggregated carriers in the carrier group.
In conclusion, in the LTE system, the existing channel transmission is generally defined with TTI=1 ms, and when another length of TTI than 1 ms is applied, the length of a TTI for downlink transmission may be changed. Since there are different transmission delays and processing delays required for data with different length of TTIs, there are different time domain positions at which ACK/NACK feedback can be made at the earliest. For this case, the method for feeding back ACK/NACK according to fixed ACK/NACK feedback timing in the existing mechanism will not be applicable any longer.